Rectifier and method of making it



3 Sheets-Sheet 1 J. H. SCAFF El AL RECTIFIER AND METHOD OF MAKING IT Original Filed Dec. 29, 1945 March 5, 1957 .J.HSCAFF- H. C. THEUEPER V B ATTORNEY INVEN Tom- BK $k March 5, 1957 J, H. SCAFF ETAL RECTIFIER AND METHOD OF MAKING IT Original Filed Dec. 29, 1945 3 Sheets-Sheet 2 J. H SCAFF H c. THEUERER AT TOR/VEV March 5, 1957 RECTIFIER AND METHOD OF MAKING IT Original Filed DeG.- 29, 1945 3 Sheets-Sheet 3 LEGEND n TYPE LOW BA c/r VOLTAGE n TYPE /-//6/-/ BACK VOLTAGE p TYPE m 'wwm r u g Mme m J. H. SCAFF MEMO H. c. THEUERER BYM ATTORNEY J. H. SCAFF ETAL I ,7 4, 58

United States Patent RECTIFIER AND METHOD F MAKIVG IT Jack H. Scalr, Summit, N. 3., and Henry C. Theuerer,

New York, N. Y., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Original application December 29, 1945, Serial No. 638,351, now Patent No. 2,602,211,v dated July 8, 1952. Divided and this application May 22, 1948, Serial No. 28,706

8 Claims. (Cl. 317-236) This application is a division of application Serial No. 638,351 filed December 29, 1945, now Patent 2,602,211, granted July 8, 1952, for Rectifier and Method of Makinglt.

This invention relates to devices that conduct electric current more readily in one direction than in the opposite direction and to methods and means of making such devices. it relates more particularly to such devices which include a body of germanium material.

Electronic asymmetric conductors of electricity may be divided into two general classes, i. e., those in which contact is made between bodies of different electrical conductivity (1) over a relatively wide area or (2) at a point or several discrete points; in either case there is a boundary condition between the bodies that inhibits current passing in one direction more than in the other. This invention is concerned with both types of condoctors, but deals more in detail with the point contact type. in devices of the point contact type, a point, usually of a metallic conductor, is pressed against a surface of a body of semiconductive material. Such devices have been called crystal detectors, or crystal rectifiers and also point contact detectors, or point contact rectifiers.

It has been found that a rectifier having particularly desirable characteristics may be made by employing a metallic point contact to a germanium body. Germanium suitable for recti-fiers may be of n-type or ptype. In a rectifier using n-type material, the greater flow of current occurs when the body or crystal is negative with respect to the point. Conversely, if the greater flow occurs when the crystal is positive the crystal material is said to be of p-type. The n-type material has a much higher rectifying capability than the p-type.

It is an object of this invention to improve thecharacteristics, particularly the electrical characteristics, of germanium type rectifiers.

A further object of this invention is the production of a germanium, point-contact rectifier capable of withstanding relatively high voltages in the reverse direction. "One 'feature'of this invention resides in the use of germanium of high purity containing controlled, extremely small amounts of certain impurities such as arsenic, antimony, phosphorus, or bismuth. For example, in one illustrative embodiment of this invention, arsenic of the order of 0.00005 percent but not more than 0.001 percent may be utilized. In another illustrative embodiment, antimony of the order of 0.001 percent but not more than 0.01 percent may be employed. Comparable percentages of phosphorus or bismuth may be used.

Another feature of the invention involves heat treatment of germanium material under such controlled temperature, time, and environmental conditions, as to produce superior characteristics for its. use. in rectifier devices.

The foregoing feature includes heat treatmentthat converts n-type germanium to p-type or p-type to n-type,

2,784,358 Fatented Mar. 5, 195? r 2 or more generally a series of heat treatments that will convert either type to the other and reconvert it if desired.

Inasmuch as the n-type germanium appears from many viewpoints to be more desirable than the p-type for rectification purposes, one feature of the inventioninvolves a casting technique that produces an ingot containing both pand n-type material, plus a heat treatment which converts the p-t'ype to n-type, thereby greatly increasing the yield of material suitable for certaintypcs of rectifiers.

The foregoing and other objects and features of this invention will be understood more clearly and fully from the following detailed description of exemplary embodiments thereof with reference to the accompanying drawing in which:

Fig. l is a sectional view of a furnace suitable for use in one stage of the process in accordance with one feature of the invention;

Fig. 2 is a sectional view of aportion of a furnace end of auxiliary means employed in another stage of the process;

Figs. 3 to 7, inclusive, are conventionalizcd sections, in accordance with the accompanying legend, of ingots of germanium. materials after different heat treatments;

Fig. 8 illustrates one form of an area contact, asymmetric conductor made of two types of germanium material, the types being indicated in: accordance with the adjacent legend;

' i'g. 9 illustrates one form of point-contact rectifier embodying this invention; and

'Fig. 10 illustrates another form of point-contact rectifier embodying this invention.

The crystals employed in the making of asymmetric conductors in accordance with this invention are cut. from suitable portions of ingots of germanium material. The ingots may be prepared from germanium dioxide in a furnace such as the one illustrated in. 'Fig. 1.. The furnace which is used in a horizontal position comprises a tube 10 of silica or like material, provided with a watercooled head 11 and a heater 12. The head 11 is provided with cooling coils 13, a cover 14 and a gas inlet 15 and is joined vacuum tight to the tube iii by pack ing 18. A shield tube 16 of silica or other suitable material is secured to the head 14 and contains a thermocouple 17. The head 14 is provided also with a gas outlet 20 and a viewing window 21.

The heater 12 may comprise a coil of resistance wire 22 wound on a suitable form 23 and having terminals 24.

The material 25 to 'be processed, in this case germanium dioxide, is contained in a dish or boat 26, which may be of porcelain or other suitable material which will not react unfavorably with the material being processed.

An illustrative reduction of germanium oxide may be.

carried out as follows: About grams of the oxide 25 are placed in a porcelain dish 26, which is put into the tube ill, which is then sealed by means of the cover 1 4. After. the furnace: tube is flushed with pure dry hydrogen, the oxide isheated to 650 C- and held at this tempera"- ture for three hours: while a: flow of hydrogen of about 10' litres per minute is maintained. During the next hour the temperature is raised to 1000 C. to complete the reduction with the germanium in the liquid state. The charge is then rapidly cooled toroom temperature. Reduction by this process results in a SI-gram body of germanium, which may subsequently be broken into lumps or pieces :of convenient size for the next step.

The next treatmentmay'be carried out in an induction furnace, portions of which are illustrated infFig. 2.. This furnace is similar to the one illustrated in Fig. '1 but ii :ZOHBS as illustrated in Fig. 4.

- greases h i d employed in the vertical position and is provided with a movable induction heater.

As shown in Fig. 2, the furnace tube 10, the lower portion only of which is shown, is surrounded by the Fc'oil 'of an induction heater. The coil 30 is provided 'witlilsuitable means for raising or lowering it with respect to the furnace charge. For example, this means may be a hoist comprising a platform 31, cable 32 and hoisting mechanism 33.

The crucible assembly, which is placed in the heating zone of the furnace tube on a bed of refractory material 34 such as aluminim oxide, may comprise a crucible 35, a graphite heater 36 and a cylinder 37 of silica or other like suitable material. The graphite heater is employed because germanium will not heat by direct induction. The cylinder 37 protects the furnace tube and reduces radiation losses.

The furnace charge may be either germanium material with the addition of about 0.1 percent tin in a porcelain or like crucible or germanium material without the tin, in a graphite crucible. After locating the crucible assembly in the furnace, the furnace tube is closed and flushed with dry tank helium. With a helium flow of one litre per minute, the charge is first liquefied and then solidified from the bottom upwardly by raising the external induction coil at the rate of /s inch per minute keeping the power input through the coil at a constant value. .After the ingot has reached 650 C. the power is shut off and the ingot is allowed to cool to room temperature. Al-

though the use of helium is preferred this step of the process may be carried out in a vacuum.

Under some conditions and for some purposes it may be desirable to perform both of the heatings in the same furnace. This could be done in a furnace such as is shown in Fig. 2. The reduction of the oxide to germanium would be done without moving the heater coil. Then the melt could either be allowed to cool and be reheated or cooled progressively by gradual removal of the heater.

Ingots made in accordance with the foregoing procedure contain germanium material which may be characterized as of three different types separated into-Zones. The three different types of germanium material, which are really only two types, p-type and n-type, with the latter arbitrarily divided into a high-back-voltage and a lowback-voltage type, are characterized by certain electrical properties. These electrical properties may be determined by making an electric probe test on a suitably prepared surface of a longitudinal section of the ingot.

An ingot made from germanium material to which 0.1

percent tinhas been added before or during melting has zones as illustrated in Fig. 3. As may be seen with reference to the accompanying legend, the bottom portion and part of the sides of the ingot which solidified first are of p-type material. The central section is of 'n-type material, which will stand a relatively high-backvoltage when used in a rectifier, and the top portion is also n-type material but of a lower back voltage. Ingots :made from germanium material processed in a graphite crucible with no tin added to the melt, are found to have In this material there is a smaller amount of p-type material, which is usually segregated into small islands near the bottom of the ingot. somewhat smaller zone at the top than is the case with germanium-tin material. The remainder of the ingot is .high-back-voltage n-type material. In both ingots, the low-back-voltage n-type material has a reverse peak voltage of the order of about 10 to volts. The high- -back-voltage n-type material ranges from 50 to volts back voltage adjacent the top to 100 to 150 volts back voltage adjacent the bottom in the germanium-tin ingots and to 100 to 475 volts back voltage adjacent the bottom {in thegermanium ingot treatedin a graphite crucible. 'lhe .p-type .materiahiwhich does not appear to be as The low-back-voltage n-type material occupies a good a rectifying material for some purposes as the n-type, will withstand back voltages of the order of about 10 volts.

Although the illustrations of Figs. 3 to 6, inclusive, show sharp lines of demarcation between zones, this is strictly true only between the p and n-type materials. Here there is an abrupt change from p to n-type across what amounts to a barrier. The n-type material near the p-type will withstand relatively high-back-voltages such as the 475 volts previously noted. As the top of the ingot is approached the back voltage becomes lower. The line shown between the two zones of n-type was arbitrarily picked at about 50 volts back voltage. The ingot contains greater amounts of impurity as the top is approached, i. e., the direction of cooling, and in some cases there is sufficient impurity near the top to give the material a low-back-voltage characteristic.

If an ingot such as those shown in Fig. 3 is heated to about 800 to 900 C. and cooled rapidly, e. g. air quenched in about fifteen minutes, most of the high-backvoltage n-type material, except for a small region adjacent the top of the ingot is changed to p-type material as illustrated in Fig. 5. By subsequently heating the rapidly cooled ingot at from 400 to 700 C. or by slowly cooling from about 800 C. the p-type germanium may be entirely transformed to the high-back-voltage n-type. Thus the germanium material may be converted into all n-type material of a high-back-voltage characteristic except for a small portion adjacent the top of the ingot as in Fig. 6.

Germanium material containing only a very slight trace of a donor impurity such as arsenic, which is later discussed, and melted in a graphite crucible with no tin added, will have all of its n-type material of the highback-voltage type. An ingot of such material when treated to convert whatever p-type material there is to n-type will be all of high-back-voltage n-type as illustrated in Fig. 7. If there is a higher amount of impurity there may be some low-back-voltage material near the top. Since specimens of both tin addition and graphite crucible types of germanium material when heated to 900 C. and rapidly cooled have substantially the same properties as those treated similarly at 800 C., the lower temperature may ordinarily be used for reasons of convenience.

In samples converted to p-type'by rapid cooling from 800 C. and reheated at lower temperatures, it was found that below 400 C. no substantial changes in the characteristics of the p-type materials are obtained for periods of treatment up to about four hours; between 450 C. and 650 C. the p-type germanium is converted slowly to strongly rectifying n-type germanium of high-backvoltage characteristic; at 500 C. or 600 C. the p-type germanium is nearly all converted to n-type in about four hours, complete transformation occurring within about twenty hours. The rate of conversion from p-type to high-back-voltage n-type germanium is dependent, upon the temperature and appears to be maximum at about 550 C. At 450 C. or 650 C. the conversion is incomplete even after twenty hours at these temperatures, whereas at 550 C. conversion is complete in aboutfour hours. if an n-type material ingot is heat treated at about 700 C. the bottom portion only is converted to p-type leaving n-type material of high-back-voltage above the p-type material. As in the original ingot there is a sharp line of demarcation between the p and n-type. Thus, a piece of material could be cut from the highback'voltage n-type zone and heat treated to make the portion which had been at the bottom with respect to the ingot of p-type leaving n-type at the top. This material could be used for making conductive devices such as shown in Fig. 8 and which are later more fully discussed.

The heat'treatment for convertingone type of, germanium to the other is completely reversible so that conversion in either direction may be obtained at will by suitable treatment. For example, the high-back-voltage n-type material obtained by means of a previously described cycle of treatment'rnay' be reconverted to p-type by heating to 800 and rapidly cooling. Also, if highback-voltage n-type germanium is desired, it may be' obt'ained directly from the ingots, as illustrated in Figs. 3 and 4 by employing the low temperature heat treatment at about 550 C. or by slowly cooling from 800 C.

To determine whether heat treatment produced a phase transformation in the germanium material, the crystal structure of p and n-type specimens prepared by heat treatment of adjacent sections from an ingot were given an X-ray examination. The structures were, however, identical, the lines obtained being in agreement to those reported in the literature for the diamond cubic germanium lattice. Subsequently precision measurements were made of the lattice constant of these same samples; but again no differences were noted;

It is believed that this invention may be understood more fully if some of the possible reasons for the behavior of the germanium material under heat treatment are discussed. Semiconductors such as are employed in dry rectifiers and like devices have been classified as excess semiconductors or as deficit semiconductors. These two types have also been called electronic semiconductors and hole semiconductors. The theory is that certain impurities in a substance upset the electronic balance of the atomic structure by the addition or subtraction of electrons. The type of conductor where electrons are added is the so-called excess type, and the impurity which gives this type of conductor is known as a donor impurity. In other cases, the impurities abstract enough electrons from the principal substance to give the unbalanced or unstable condition which causes current to flow. This is the deficit type of semiconductor and the impurity which causes the deficit by abstracting electrons is known asan acceptor impurity.

Before further discussion of the effect of the donor and acceptor impurities, it may be Well to note that the germanium materials under consideration in many respects behave similarly to precipitation hardening alloys. Such alloys contain a constituent whose solid solubility increases with increasing temperature. If such an alloy of a given composition is then heated above the solubility temperature, the solid solution may be retained in a; metastable state at room temperature by cooling rapidly. On reheating to a temperature below the solubility temperature, the unstable solution decomposes,

precipitating a new phase from solution with resultant changes of physical and electrical properties; In the present instance the formation of p-type germanium by rapid cooling may result from the retention of an impurity in solution while formation of n-type germanium may result from precipitation of this impurity from solu tion.

The known donor impurities for germanium are arsenic, antimony, phosphorus, and bismuth, or in other words, the members of the odd series of the fifth periodic group according to Mendeleetf. It has been found that material considered to be essentially pure germanium contains very small amounts of arsenic and phosphorus. Such material has been successfully used for making germanium crystals for rectifiers of n-type rectification.

However, when these impurities were removed no n-type rectification was obtained regardless of the treatment of the material. Furthermore, it has been found that if proper impurities are added to this material from which impurities have been removed, n-type rectification can be again obtained. For example, small amounts of arsenic, antimony, and bismuth have been added to such materials with satisfactory results. In view of the foregoing it seems reasonable to believe that the donor impurities previously noted are responsible for n-type rectification of germanium. It seems probable that an acceptor impurity exists which tends to produce p-t'ype rectification.

'In' the region: of the ingot which is n-type, the donors areinexcess and in the region of the ingot which. is ptype; acceptors are in excess.

To explain the inversion'in the rectification of geruranium which occurs on heat treatment, it is necessary to assume that either the donors or the acceptors can be thermally activated. Assuming that the acceptors are activated by heat treatment at 800 C; and are retained in this state by" rapid cooling to room temperature, then those portions of the ingot which have a higher concentration of active acceptors than of donors, will have p-type rectification.

Because of differences in the rate of segregation of two impurities on solidifying an ingot" from the bottom upward, the relative concentration of the impurities may vary at progressive locations in the ingot. For example, the donor may have a lower concentration at the bottom of the ingot thanthe acceptor, and still be in excess of the acceptor higher in the ingot. Under these conditions an ingot, heat treated as above, may have p-type rectification near the bottom where the acceptor is in excess, and n-type' rectification at locations higher in the ingot where the donor is in excess. The line of demarcation between. p and n-types is sharply defined. This atfords an explanation for the shell of p-type germanium found in certain initial ingots before further heat treatment. Evidently the cooling conditions, which occur in the normal cooling cycle, are such that an excess of active acceptor is presentin the first material to freeze and the material is p-type. Due to differences in segregation rates, as freezing progresses, the donor concentration rapidly overtakes the active acceptor concentration and an inversion to n-type germanium occurs. If after the 800 C. heat treatment, the ingot is subsequently heated at about 500 C. the acceptors are deactivated and in consequence the donor impurities are in virtual excess throughout the ingot and only n-typerectification is observe-d.

An alternative explanation of the heat treating phenomena involves the assumption that the donors are deactivated by appropriate thermal treatment. The reasoning is analogousin the former case except that now the 800 C. treatment deactivates the donors and rapid cooling retains their inactive form, while subsequent heating atabout 500 C. results in conversion to the active form. To explain. complete" conversion to n-type germanium by the 500 treatment, it is now necessary to postulate that the active donor concentration is everywhere in excess of the acceptors. Since in some cases the 800 C. treatment results in only a partial conversion of p-type germanium, it is necessary to-suppose that at high concentrations the donors areincompletely deactivated.

Although both of these explanations are in agreement with the experimental evidence, the concept of thermally deactivated acceptors is preferred, because it is compatible with the solid solubility concept previously referred to. In general, it has been observed that impurities which form solid solutions with semiconductors, reduce their resistivity and tend to' produce strongly rectifying materials. Since p-type rectification is observed in ingots rapidly cooled from 800, it seems reasonable that theta"- ceptors, which are held in solid solution by this process are activated. The conversion to n-type germanium by heating at 500C. may then be due to the deactivation of the acceptors by precipitation from this unstable solid solution.

Although no acceptor impurity" has been definitely identified, it is reasonable to suspect that this impurity, or

one of them may be oxygen. Thermal transformations are known to occur in germanium oxide near 500 C. Moreover, ingots made in graphite crucibles have much less tendency to form p-type germanium, and this may be due to an initially lower oxygen content in consequence of the reducing nature of the graphite. Furthermore, since the tin in the germanium-tin composition. has been found to perform no function in the finished material (Fig. 6) by subsequent heat treatments. due to additional donor impurities carried by the tin which a rectifier device.

from the electricalviewpoint, most of the tin being segregated in the generally unusable top portion of the ingot, it seems reasonable to believe that tin may also act as a deoxidizer although not to the same extent as the graphite.

Since the presence of p-type germanium is thought to be due to an acceptor impurity, such as oxygen, it follows that if this impurity could be completely eliminated from the ingot, only n-type germanium would be found, irrespective of the thermal history of the ingot. On the other hand, the concentration of the donor impurities might be increased sufiiciently without changing the acwas added or to intentionally added impurity.

After processing, the ingot of germanium material may ,be cut into small bodies or crystals for use in rectifiers.

Although the preferred material is the n-type high-backvoltage material, since it may be used to make a rectifier of highrectification ratio, the other material will also make Also, if an ingot such as is shown in Fig. 3 be cut so that a slab or body contains both 11 and p-type material and area-contact or volume type rectifier such as disclosed in Fig. 8 may be made. In this figure,

the slab is made up of a portion 40 of nigh-back-voltage n-type material, and a portion 41 of p-type material separated by a barrier 46, electrodes 42 and 43 are secured respectively to the two sides of the device, and leads 44 and 45 secured, as by soldering for example, to the respective electrodes. Besides being a rectifier the device illustrated in Fig. 8 also exhibits photoelectric properties when irradiated at the boundary 46 between the two types ,of germanium at 40 and 41.

As previously noted, an n-type slab may be suitably treated to obtain a conductor like this.

One form, which the point-contact type of rectifier may take, is illustrated in Fig. 9 in which a main housing 50 of a ceramic or like insulating material is provided with .metallic end pieces or members 51 and 52, which screw into the opposite ends of the housing 50. The rectifier elements are carried on the respective ends of pins 53 and S4, fitted into bores in the end pieces 51 and 52. A crystal element 55, which may be metal-coated on one side, for example with copper, is secured to the end of the pin 53, which may be of brass, and an S-shaped contact spring 56 is secured to the end of the pin 54, which may also be of brass. The spring contact 56 may be of tungsten suitably pointed at the end which makes contact with the crystal 55. The parts are adjusted by suitable positioning of the pins 53 and 54 and are then held in place by means of the set screws 57 and 58. The adjustments are carried on along with electrical stabilizing until the device exhibits the characteristics desired for a particular purpose. After the adjustments are completed,'the units are vacuum impregnated with a suitable mixture such as a wax through the orifice 59 in the body 50. Connection may be made to the reduced portion of the end pieces 51 and 52 by means of friction type connectors 60 and 61.

A sealed unit similar in some ways to Fig. 9 is shown in Fig. 10. A body of phenolic condensation product or like material has sleeves or cylinders 71 and 72 molded Lin opposite ends thereof. Studs 73 and-74 of brass or like material, which are a press fit in the sleeves 71 and 72, carry respectively the crystal 75 and the springcontact element 76. The studs 73 and 74 are forced into the .sleeves 71 and 72, respectively, in positions for suitable operation of the device, as determined by appropriate electrical measurements. The studs are then secured in their respective sleeves by means of solder as shown at 77 and 78, respectively. The device may be vacuum impregnated with a suitable wax or wax-like mixture through the orifice 79. Connectors 80 and 81 making a frictional fit with or otherwise secured to the sleeves 71 and 72, respectively, may be used for making electrical connection to the rectifier device.

'In a device such as is shown in Fig. 9 low capacity across the unit is obtained by means of low dielectric constant insulating material and small diameter of the parts.

, Crystal elements, such as 55 and 75 of the devices shown in Figs. 9 and 10, respectively, may be prepared for use before assembly by lapping the surface to which contact is to be made on a suitable smooth surface with a fine abrasive. The surface may then be etched. A suitable etchant may comprise 10 cubic centimeters of nitric acid, 5 cubic centimeters hydrofluoric acid and 200 milligrams of copper nitrate in 10 cubic centimeters of water. An etching in such a solution for about thirty seconds gives a suitable surface.

The active surface of the crystal element may also be subjected to an electrolytic etching, which improves the device for some purposes by considerablyreducing the back current. This etching may be done after the nitrichydrofiuoric etching previously noted, or may be done di rectly on a lapped crystal Without the intermediate etching. The crystal may be etched at a positive potential of from 4 to 6 volts direct current for from 30 to seconds in 24 percent hydrofluoric acid.

Although specific embodiments of the invention have been shown and described, it will be understood that they are but illustrative and that various modifications may be made therein without departing from the scope and spirit of this invention as defined in the appended claims.

Reference is made to application Serial No. 28,707 filed May 22, 1948, now Patent 2,603,692, granted July 15, 1952 which discloses related subject-matter.

What is claimed is:

1. An asymmetrical conducting device comprising a body consisting essentially of germanium containing in its atomic lattice a trace, not more than .001 percent, of donor impurity from the group consisting of arsenic and antimony, and conductive means in contact witha sur face of said body. a

2. An asymmetrical conducting device comprisinga body consisting essentially of germanium containing in its atomic lattice a trace, not more than .001 percent,

of donor impurity from the group consisting of arsenic and antimony, and a metallic point in contact with a lapped and electrolytically etched surface of said body. 3. An asymmetrical conducting device comprising a body consisting essentially of germanium containing in its atomic lattice, a trace, not more than .00005 percent, of donor impurity consisting of arsenic, and means for making electrical connections to spaced portions of said body.

4. An asymmetrical conducting device comprising a body consisting essentially of germanium containing in its atomic lattice, a trace, not more than .00005 percent, of donor impurity consisting of arsenic, and means for making electrical connections to spaced portions of said body, one of said means comprising a point contact;

5. An asymmetric conducting device comprising-a body of germanium containing a trace of antimony,'of the order of .001 percent, and means for making electrical connections to spaced portions of said body.

6. An asymmetric conducting device comprising a body of germanium containing a trace of antimony, of the .order of .001 percent, and means for making electrical connections to spaced portions of said body, one of said means comprising a point contact.

7. An asymmetrical conducting device comprising a body consisting essentially of germanium containing in its atomic lattice a trace, not more than .001 percent, of donor impurity consisting of arsenic, and conductive means in contact with a surface of said body.

8. An asymmetrical conducting device comprising a body consisting essentially of germanium containing in its atomic lattice a trace, not more than .001 percent, of donor impurity consisting of arsenic, and a metallic point in contact with a lapped and electrolytically etched surface of said body.

References Cited in the file of this patent UNITED STATES PATENTS 2,378,513 Thompson et al. June 19, 1945 10 Scafi June 25, 1946 Ohl June 25, 1946 Olsen Feb. 22, 1949 Woodyard Nov. 14, 1950 Olsen Ian. 22, 1952 OTHER REFERENCES Merrit: Proc. National Acad. of Science, vol. 11, 1925, 

2. AN ASYMMETRICAL CONDUCTING DEVICE COMPRISING A BODY CONSISTING ESSENTIALLY OF GERMANIUM CONTAINING IN ITS ATOMIC LATTICE A TRACE, NOT MORE THAN .001 PERCENT, OF DONOR IMPURITY FROM THE GROUP CONSISTING OF ARSENIC AND ANTIMONY, AND A METALLIC POINT IN CONTACT WITH A LAPPED AND ELECTROLYTICALLY ETCHED SURFACE AND SAID BODY. 